Simple genetics example

1
Simple genetics example

• Albino rabbits have white coats and red eyes.
• This is due to lack of the pigment melanin
• Albinos are actually rather rare in the wild, but
  common in pet shops!
• The original albino rabbit arose as a result of a
  mutation a very rare event due to a change in
  the DNA of the coat colour gene.

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Simple genetics example

• Brown rabbits are the normal colour in the wild.
  The pigment melanin gives them colouration which
  enables them to blend in with their surroundings,
  and protects them from UV radiation.

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Parents

• We shall consider what happens if we cross an
  albino rabbit with a brown rabbit.
• It does not matter which one is the male, and
  which is the female
• The result is the same

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F1 offspring

• All the offspring are the same they look brown

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Using F1 offspring as parents

• If two of these rabbits are allowed to reproduce

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F2 offspring

• The result is
• most of the offspring are brown
• some are albino

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F2 offspring

• Actually the proportion is
• 3 brown 1 albino
• (as a ratio)?
• 75 brown, 25 albino
• (as a percentage)?

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F2 offspring

• We can also use these figures to give the chance
  of an individual rabbit being one colour or the
  other

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F2 offspring

• There is a 3 in 4 (¾) or 75 chance of any baby
  rabbit being born brown

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F2 offspring

• There is a 1 in 4 (¼) or 25 chance of any baby
  rabbit being born albino

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Some genetics terms(dominant/recessive)?

• Since all the F1 offspring are of one type, we
  have a straightforward situation.
• We use the term dominant to describe the
  characteristic seen in the F1, and most of the F2
  brown
• Because they seem to recede and are not seen in
  the F1, we use the term recessive for the albino
  forms

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Some more genetics terms(phenotype/genotype)?

• Phenotype is a word used to describe an organism
  by its observable characteristics, i.e. what it
  looks like, and we use ordinary words to describe
  it
• Genotype is defined at the genetic level, i.e.
  what genes it contains. This is usually not
  directly visible, and we describe it using a code

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Some more genetics terms(genes/alleles)?

• In rabbits, the two colour forms are caused by 2
  versions of a gene.
• We say that the coat colour gene has 2 alleles
  brown and albino

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Genetics conventions

• It is normal to choose a particular letter to
  stand for the gene we are considering (could be A
  or B).
• We use upper case (CAPITALS) for the dominant
  form and lower case for the recessive form

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Genetics conventions

• The original brown parent is described using the
  genotype BB (could be AA)?
• The original albino parent is described as bb
  (could be aa)?
• These letters are in pairs because chromosomes
  are in pairs in adult cells

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Genetics explanations

• The brown rabbit (genotype BB) produces gametes
  (sperms or eggs, depending on its sex) containing
  only the allele B.
• The albino rabbit (genotype bb) produces gametes
  (eggs or sperms, depending on its sex) containing
  only the allele b.
• This is because meiosis separates chromosomes so
  that only one of each set is present in the sex
  cells

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Genetics explanations

• The brown rabbits of the F1 all have
• the genotype Bb.
• Their phenotype is brown because they contain a
  dominant allele B, and only one dominant allele
  is enough to give this colouration.
• They have the same phenotype as their parent
  (brown) but a different genotype.

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Genetics explanations

• The white rabbits of the F2 all have
• the genotype bb.
• Their phenotype is white because they contain a
  pair of recessive alleles bb, and two recessive
  alleles are required to give this colouration.
• They have the same phenotype as their grandparent
  (white) and the same genotype. Only double
  recessive genotypes can be forecast this way.

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Genetics more terms

• Because rabbits with the genotype Bb contain two
  different alleles they are called heterozygous,
  whereas rabbits with the genotype BB and bb are
  both called homozygous or true-breeding.

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Genetics predictions

• In heterozygous brown rabbits the recessive
  allele b is hidden, but it could reappear in the
  next generation.